Unlock Oni Kid
To unlock Onimusha Tactics' only hidden character, you need to complete All 16 Floors of Phantom Realm.

Easy Experience
Find a stage that your character could first fight with an enemy, or any stage that your will touched the enemy before the enemy fights you. Then, choose the lowest level character from your team, and train it in the stage. For example, after you just touched the enemy and gain the little bit experience (about 48 experience points), and just before all your team members end their turn, escape from the match. Then start over. Continue doing to earn a lot of experience.